"rbm fetch" should accept options to select target

Description

The rbm fetch command is used to fetch new commits from git/hg repositories. It currently takes the following options:

rbm fetch [project]

In tor-browser-build we have a single make fetch command fetching commits from all components, which should work for any type of build (release/alpha/nightly) as they currently all use the same git repositories.

In #24179 however we are thinking about using a different git repository for releases and the nightly build, which means rbm fetch needs to know which type of build we want to fetch, so it should support --target options.